Current expansion joints in gas turbine exhaust systems with hot flanges are generally designed in one of two ways. The most common design is a flat belt arrangement consisting of a ceramic fiber composite belt and bolster bag. One end of the belt is bolted to a frame attached to the gas turbine and the other end is bolted to a frame on the adjacent exhaust ductwork. The gas seal in the composite belt is provided by a Teflon.TM. layer, metal foil or metal impregnated cloth. The other most common design, sometimes called a "D" style due to the cross-sectional shape of the belt, is a composite belt and bolster with one end bolted to a bar mounted on the gas turbine flange and the other end bolted to a flange on the exhaust ductwork. The belt acts as a diaphragm.
The first mentioned flat belt arrangement has experienced problems including cracked frames due to thermal transients; burned belts due to frames cracking and bolster bag failure; and leakage during gas turbine water wash cycles which not only allows contaminated water to leak onto the ground, but also damages the ceramic fibers in the belt and bolster bag. Liquid fuel ingestion in the event of a failure to fire on liquid fuel-fired gas turbines damages fibers and binders and burns when gas turbine exhaust temperature increases. Moreover, while this design handles large relative motions (3 or more inches axially), the ceramic fiber, once stretched, does not return to its original shape, thereby resulting in burning and hot gas leaks.
The second mentioned "D" style composite belt and bolster bag arrangement has also proven less than satisfactory in that it is very difficult to install this style belt on the ductwork, and because, over time, the bolster bag will degrade and allow hot gas to damage the belt, possibly also resulting in failure.
A metal bellows arrangement has also been used in similar applications, but requires two bellows separated by several feet to accommodate lateral offset as well as relative axial movement. In some cases, the metal bellows concept requires high quality field welding. Overall, the metal bellows technique for large gas turbines have been susceptible to cracking due to thermal fatigue and is thus also unsatisfactory.